This invention concerns a magnetooptic recording medium for recording and reproducing information using laser beams and, more specifically, as it relates to a magnetooptic recording medium comprising an amorphous layer of rare earth-transition metal formed on a plastic substrate for use in E-DRAW recording material capable of erasing and re-recording, that is, repeating recording.
There has been known a magnetooptic recording method in which a rare earth-transition metal amorphous layer is formed on a substrate, for example, by means of vacuum deposition, sputtering or ion plating, and information is written and erased through the thermomagnetic effect by irradiating laser beams to the amorphous layer and reading the information through the magneto-optical effect. As the substrate for the amorphous layer, while there have been proposed glass plates, metal plates such as of aluminum or plastic plates such as of acrylic resin and polycarbonate resin, prior study has mainly directed to the formation of the amorphous layer on the glass plate. Although it is possible in the case of the glass substrate to form a vertically magnetizable amorphous layer thereon, a technique of forming a vertically magnetizable amorphous layer on the plastic substrate in a stable state has not yet been established at present. Specifically, a Kerr rotational angle of greater than 0.3 degree can be obtained in the case of the glass substrate. However, in the case of the plastic substrate, Kerr rotational angle only of about 0.1 degree can be obtained. While the angle may be increased to more than 0.2 degree in an exceptional case, it lacks in the long time stability and has poor reproducibility. In view of the above, it has been highly demanded to establish a technique for forming a vertically magnetizable amorphous layer stably on a plastic substrate easy to handle with instead of the glass or aluminum substrate employed in the prior art.
It has been considered that the vertical magnetization characteristic of the rare earth-transition metal amorphous layer is dependent on the distortion in the amorphous layer (refer to H. Takagi, et al., J. Appl. Physics 50 (3), pp. 1642-1644). Accordingly, the surface property of the substrate on which the amorphous layer is formed constitutes a significant factor. Although it is not apparent at present why the rare earth-transition metal amorphous layer capable of vertical magnetization can be formed on the glass plate but not stably on the plastic plate, the present inventors have considered that the reason is basically attributable to the following two factors.
(1) Acrylic or polycarbonate resins employed as the plastic substrate generally have lower surface energy as compared with that of the glass substrate and thus poor bondability with the rare earth-transition metal amorphous layer and, as the result, it is difficult to produce distortion in the amorphous layer.
(2) Oxygen, moisture, monomer and the like remain in the plastic plate and they deteriorate the rare earth-transition metal amorphous layer to impair the vertical magnetization characteristic thereof.
In view of the above, a method of forming a moisture-proof inorganic protection layer, for example, made of SiO.sub.2 and CaF.sub.2 has been proposed in order to solve the problem (2) (refer to, for instance, Japanese Patent Laid-Open Nos. 27451/1982 and 32238/1983). However, the problem (1) cannot be solved by such provision of the protection layer. That is, no rare earth-transition metal amorphous layer having stable vertical magnetization characteristic can be formed on plastic substrates.
Accordingly, the object of this invention is to provide a process for forming a rare earth-transition metal amorphous layer on a plastic substrate.